The present invention relates to an imaging assembly that comprises photohardenable microcapsules containing coloring agents. Development is accomplished by the application of uniform pressure to the imaging assembly. In particular, the imaging assembly comprises an opaque support having a preselected vapor transmission rate in combination with optical properties that provide for improved image quality and performance.
The present invention relates to an imaging medium in the form of a self-contained imaging assembly and, more particularly, to an improved self-contained imaging assembly containing a photosensitive imaging layer or layers comprising photohardenable microcapsules encapsulating a coloring material and, outside the microcapsules, a developer material disposed between a first transparent support and a second support which may be opaque or transparent. The imaging medium or assembly can also be referred to as a recording medium, and the imaging layer can be referred to as a recording layer, since the assembly can serve both to capture an image (either the original image or an electronic copy), as does film, and also to display the image, as does a print. Consistent with this fact, the imaging assembly can form a positive image.
The photosensitive imaging layer (including microcapsules) is colored by pressure development after exposure to radiation based on image information. The microcapsules, whose mechanical strength changes (increases) when exposed to light, are ruptured by means of pressure development, whereupon the coloring material and other substances encapsulated in the microcapsules flow out (to varying amounts based on the exposure) and development occurs. The coloring material, such as a substantially colorless color former, migrates to, and reacts with, the developer material and coloring occurs, whereupon a color image is developed.
The xe2x80x9crupturexe2x80x9d of the microcapsules are not an all-or-nothing event. Rather, the microcapsules exposed to light are differentially photocured to release varying amounts of color former in order to achieve tonal depth in the exposed area. The differential exposure to light proportionately increases the viscosity of the photocurable composition and thus immobilizes the color former proportionately to the desired tonal depth in the exposed area. The rupture of the microcapsules and the release of the color former is accomplished by the uniform application of pressure. Development of the photosensitive imaging layer can be accomplished, for example, by passing the imaging assembly between a pair of upper and lower nip rollers.
Photohardenable imaging systems employing microencapsulated photosensitive compositions are the subject of various patents, including U.S. Pat. Nos. 4,399,209, 4,416,966, 4,440,846, 4,766,050, 5,783,353, and 5,916,727. Image forming devices (also referred to as printers) are disclosed, for example, in U.S. Pat. No. 4,740,809, wherein exposure occurs by guiding a light from a light source for a plurality of colors across a photosensitive recording medium. U.S. Pat. No. 4,992,822 discloses an image forming device, capable of producing a plurality of colors via a polygonal mirror, for repeatedly exposing the same pixels in a photosensitive recording medium. U.S. Pat. No. 5,893,662 discloses a device for printing an image wherein the device can be incorporated into a computer bay. U.S. Pat. No. 4,648,699 describes a development technique which employs, instead of a pair of nip rollers, a point contact ball moving relative to the photosensitive recording medium.
In the most typical embodiments, the photohardenable composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated together with a color former. Exposure to actinic radiation hardens the internal phase of the microcapsules. Then, as mentioned above, following exposure, the imaging media in the form of a sheet can be subjected to a uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers.
One of the problems in providing a self-contained imaging assembly that provides a high quality print is image stability or xe2x80x9ckeepingxe2x80x9d which is affected by humidity sensitivity. It is known that print quality, and in particular sensitometric response to actinic radiation, can be significantly affected by sensitivity to humidity and the relative humidity of the environment. Even if the media is manufactured and packaged at a particular humidity, which is found optimum for print quality, variations after the media acclimates to a different later environment can adversely affect the sensitometric properties. This has been believed due to the materials employed in the imaging media, in particular the degree of hardening or curing of the internal phase of a microcapsule and the consequent increase in the viscosity varying with a change in humidity. As a result thereof, photographic characteristics such as speed, maximum density and fogging density are changed from the original optimum. Furthermore, a full color imaging is adversely affected.
In forming a full color image, color precursors which develop into yellow, magenta and cyan colors and photo-initiators corresponding to blue, green and red lights are encapsulated in an internal phase of the microcapsules, and the three sets of the microcapsules are mixed to prepare a full color imaging material containing a developer. The photographic characteristics of the respective microcapsules vary with a change in humidity to different degrees, resulting in muddy colors or incorrect or suboptimal colors. For example, when it is desired that a yellow color be developed, cyan and magenta capsules are cured by red and green lights, and only a yellow color former reacts with a color-developer to form an image. However, if the cyan or magenta capsules insufficiently cure due to a change in humidity, the result may be a muddy color in which cyan or magenta is blended with yellow to some extent. Such muddy colors or other sensitometric phenomena due to a change in humidity has been a significant problem.
One technique that has been used to address the humidity problem and to improve media stability resides is conditioning the layer containing the developer and microcapsules to a relative humidity (RH) of about 10 to 40% and preferably about 20%. For example, U.S. Pat. Nos. 5,916,727 and 5,783,353 disclose conditioning the layer at about 20% RH for about 2 to 12 hours or more, at ambient temperatures, and subsequently sealing the assembly at this low RH level to assure that the layer is relatively moisture-free during the normal shelf-life of the assembly.
U.S. Pat. No. 5,996,793 discloses storing the image-forming material together with a humidity-controlling material. Further, the patent discloses storing the image-forming material and the humidity-controlling material within a package made from a low-moisture permeable film. The low-moisture permeable film can be a plastic film on which is deposited a metal. Other low-moisture permeable films mentioned include fluorinated resins such as polytetrachloroethylene, polytrifluoroethylene, chlorinated rubber, polyvinylidene chloride, a copolymer of polyvinylidene chloride and acrylonitrile, polyethylene, polypropylene, polyesters, and films obtained by depositing a metal such as aluminum and a metal oxide such as silicon oxide.
Unfortunately, when the imaging media are not used right away, especially if a plurality of media are stored for some time in a printing device prior to forming an image, the media may have an opportunity to adjust to ambient humidity and, especially in very dry or very humid climates, the RH of the media may decrease or increase substantially in a short time. Once the imaging media is removed from a package, it does not take very long for the environmental humidity to affect the media. Ambient humidity can soon penetrate the outside surface support on each side of the media cause a change in the moisture content within the media.
U.S. Pat. No. 5,783,353 teaches that commercially available materials may be used for the opaque support in media comprising light-sensitive, pressure rupturable microcapsules. Materials listed include paper, cardboard, polyethylene, and polyethylene-coated paper. Opaque films are typically composites or admixtures of the polymer and the pigment in a single layer. U.S. Pat. No. 5,783,353 states that, alternatively, an opacifying agent can be provided in a separate layer underlying or overlying a polymer film such as PET. The patent further states that the opaque support should be sufficiently opaque so that when an imaging medium is exposed to radiation through the transparent support, the opaque support is effective to prevent the radiation from penetrating to other imaging media which may be stacked behind the imaging medium during the exposure step. The patent states, however, that if the units are not exposed in a stacked format, the opacity of the support is not critical so long as the support provides the desired background.
Metal barriers have been used in other contexts, in the photographic and packaging arts. In the packaging art, excellent oxygen and moisture barrier properties are desired or required. One approach to improving the oxygen and moisture vapor transmission of polymer films in the packaging art, in particular, is the application of a barrier coating in the form of a metal coating to the surface of a packaging film, to thereby form a metallized film.
U.S. Pat. No. 6,033,786 teaches a biaxially oriented, heat-set, multilayer film, including a core layer, a bonding layer having a surface adhered to the core layer, and a flame treated surface opposite the surface adhered to the core layer. A metal coating for providing oxygen and moisture barrier properties is deposited on the flame treated surface and a protective plastic film adhered to the metal coating. The core layer and bonding layer either are free of void-creating additives or include only a quantity of such additives that does not create a matte surface adversely affecting the barrier properties of the metal coating. The bonding layer comprising a mixture including 40 to 100% by weight of propylene/butene-1 copolymer containing up to about 14% by weight of butene-1, 0 to 60% of an isotactic polypropylene, and 0 to 50% of a copolymer of ethylene and propylene wherein propylene is the predominant component by weight.
Numerous patents disclose various methods of coating a polymer film with a metal. For example, Japanese Pat. Publication No. 61-225049 teaches preparation of metallized polypropylene films by extruding a mixture of polypropylene and a hydrocarbon resin and treating the surface thereof with corona discharge in a nitrogen/CO2 atmosphere. This treatment inserts amino and/or amido groups into the surface of the film to a depth of about 100 Angstroms. The thus treated film surface is then metallized. U.S. Pat. No. 4,888,237 discloses a metallized film in which the surface that is provided with a metallized coating is the surface of a crystalline polyolefin layer that is flame treated prior to being metallized. U.S. Pat. No. 4,487,871, issued to Ishibashi, et al., discloses a polyolefin resin composition including a propylenebutene-1 copolymer having a propylene component in a weight percent of at least 70%. This copolymer is present in the range of 96%-80% by weight, and the composition additionally includes 4% to 20% by weight of a high density polyethylene homopolymer or copolymers of ethylene as a main component with other alpha-olefins. U.S. Pat. No. 4,487,871 discloses corona treating a surface of the composition to be provided with a metal coating.
After extensive investigation, Applicants have found that humidity affects the mechanical properties of the imaging layer in a self-contained imaging assembly comprising photohardenable microcapsulated coloring agents, as compared to affecting the reaction properties of materials during photohardening of the imaging layer. This effect on mechanical properties causes undesirable variations in the degree of rupture of the microcapsules when the media is subjected to pressure during development. Although not wishing to be bound by theory, this may be due to the humidity changing the break strength of the capsules and/or it may be due to the change in pressure applied on the capsules due to the change in the stiffness of the image layer which in turn increases the stress applied to the microcapsules, more likely the latter.
An object of the present invention is, therefore, to provide a self-contained photohardenable imaging assembly that is resistant to the affect of humidity and which will print consistently in response to a means for applying pressure to the assembly.
Another object of the present invention is to provide a self-contained photohardenable imaging assembly that provides better image quality by producing greater whiteness, opacity and reflectance.
Still another object of the present invention is to obtain an improved media that has no significant change in sensitometric properties with relative humidity, based on speed, Dmax, Dmin, tonal scale, and full color correctness.
Still another object is to obtain an improved media that has improved Raw Stock Keeping (RSK), from manufacture to use.
It would be particularly desirable if these objectives could be accomplished without requiring radical changes in conventional imaging chemistry, with respect to the microcapsules and the developer. It would be advantageous if these objectives could be attained in a product that was economical to manufacture and inexpensive for the customer to purchase.
In the self-contained imaging assembly of the present invention, an imaging layer containing developer and photohardenable microcapsules is placed between two support members to form an integral unit, wherein one support is transparent and one support is opaque, comprises a metallic barrier layer, and exhibits a water vapor transmission rate of less than 0.77 g/m2/day (0.05 g/100 in2/day). The opaque support also has very high opacity and reflectance at the interface with the imaging layer. Preferably, the imaging assembly is also sealed on the sides to further prevent water vapor from permeating out of the imaging layer. The term xe2x80x9csealed,xe2x80x9d as used herein, refers to a seal which is designed to be non-temporary. This seal is maintained during printing of the image and in the final imaged product, as compared to a temporary package.
In the imaging assembly of the invention, a first support is transparent and a second support is opaque. Consequently, an image is provided against a substantially white background as viewed through the transparent support. Sometimes herein the first support may be referred to as the xe2x80x9cfrontxe2x80x9d support and the second support may be referred to as the xe2x80x9cbackxe2x80x9d support.